Geothermal energy extraction systems, such as geothermal power generating systems, are known as renewable energy sources with relatively low carbon emission footprints. A traditional geothermal power system generally may be classified in one of three conventional types, as follows:
(1) Dry steam power plants, which capture and use steam exiting from natural fissures in sub-surface rock, where deeper water has been boiled into steam via sub-surface heat, to drive a power generation turbine, with the turbine typically producing electricity. Dry steam power plants, however, are typically limited for use in areas that have naturally occurring conditions that are suitable for such use, and therefore there are relatively few dry steam power plant installations.
(2) Flash steam geothermal power plants, in which high-temperature and high-pressure water (typically at water temperatures around, or greater than, 180° C.) is artificially pumped (via large mechanical water pumps) under high pressure from typically deep wells to the surface. Upon reaching the surface, all or a portion of the very hot and high-pressure water is flashed into steam that may be used to drive turbines and/or generators.
For some flash steam plant installations, wells are drilled into natural brine water basins, with the brine water being pushed to the surface and out of the well by the naturally occurring elevated pressure of the brine water at depth, similar to some oil wells that do not require pumps because of the pressure at depth that naturally pushes the oil up and out of the well. However, in such flash steam plant systems, depletion of the original sub-surface water table is a serious environmental concern, as a significant amount of the sub-surface water may not condense and return to the basin, thereby necessitating the use of large-scale re-injection pumps to force the remaining condensation back down a separate return well.
Flash steam plants also use transport lines that are susceptible to mineral scaling by the brine water, which is an extremely significant problem that may necessitate frequent and costly de-scaling operations.
Still further, there are only a limited number of geographic locations that have conditions suitable to sufficiently pressurize water to produce viable amounts of electrical power, and it is economically infeasible to install and operate a flash steam plant in some of these locations. To be economically feasible for power production, the temperature of the hot water exiting the well may need to be 180° C. or more, and it is often costly to access sub-surface locations where the necessary heat and pressure naturally occur. Further, the use of large water pumps to pump water into and/or out of the well results in a significant parasitic load of the well's power production capacity.
For example, while it may be possible to access an area where the necessary temperature naturally occurs by drilling a deep enough well, such extremely deep wells would typically need to be about eight kilometers deep, as the temperature in the earth's crust reportedly increases about 22-27° C. per mile of depth from the surface. There are areas in the world where molten magma is found closer to the surface, and such areas are referred to as geothermal “hot spots”. However, such “hot spots” are relatively rare and are sought after for use by flash steam plant systems. To be economically viable, such “hot spots” often need to provide sufficiently heated water/brine within approximately 3 kilometers of the earth's surface.
To provide economically viable working fluid pressure differentials for the turbine and generator, which are situated between the geothermal heat source and the cooler/condenser utilized in such systems, the water must be super-heated by a significant temperature level that is typically found only at extreme depths. When a column of water is subjected to extreme depths, the water pressure increases at approximately 0.433 pounds per square inch (“psi”) per foot of depth. The elevated pressure at great depths prevents extremely hot water from boiling as it circulates into and out of a very deep well. Therefore, at least one (and typically two) large, power-consuming, water circulating pump(s) is/are necessary, among other equipment, for a flash steam power production plant. Also, as mentioned, the scaling of minerals on the interior walls of the brine water transport lines poses a serious maintenance cost issue, since, unless the continuous build up of scaling is removed, the design brine water flow is restricted and design flow rates and corresponding power production levels are impaired.
(3) Binary cycle power plants may operate at lower temperature ranges than conventional flash steam plants, but ultimately they also drive a power generating turbine that typically drives an electrical generator. In a binary cycle power plant, hot water or steam that is heated geothermally or by waste heat (such as from nuclear or fossil fuel power plants, or the like) is transported by a pump through a first primary water loop to a heat exchanger. The heat exchanger transfers most of the heat to a secondary closed loop that is used for power production.
Conventional binary geothermal power systems use at least one water loop (typically with an open loop at the bottom of the well) that circulates water within a geothermal heat source to acquire heat. That heat is then exchanged, in an above-ground heat exchanger, with a separate above-ground refrigerant loop used to generate power. Most conventional geothermal power systems are classified as binary systems, and typically require at least two primary heat exchange loops and at least two (and usually at least three) fluid circulating pumps.
In what is commonly referred to as an enhanced geothermal system (EGS), at least two separate wells are drilled a sufficient distance apart. Next a special fluid (often containing dangerous chemicals) and/or a pressurized gas is force-pumped into at least one of the wells so as to fracture the sub-surface hot rock (similar to “fracking” in the oil and gas industry). Then, pumps circulate the primary water/brine working fluid into and out of each respective well, with at least one well and pump being utilized as a return water/brine well and pump (so water, with heat extracted by and within the water to refrigerant heat exchanger, can be pumped back down into the sub-surface geology to regain heat), and with at least another well and pump being utilized as a supply water/brine well and pump, so geothermally heated water/brine can be pumped up to the surface for circulation through the heat exchanger that transfers heat to the secondary working fluid power generation loop.
At the surface of either a traditional binary system or an EGS design, the heated water is sent through a heat exchanger (typically a plate to plate heat exchanger or a tube within a tube heat exchanger, or the like), where the heat within the water/brine is absorbed and removed by a colder secondary working fluid having a materially lower boiling point than water, such as a refrigerant (e.g., R-134A), propane, or the like. Heat naturally flows from the hot primary working fluid (water/brine) into the colder secondary working fluid (refrigerant), as heat naturally flows to cold (Fourier's Law).
Although the primary working fluid water/brine is very hot (usually, but not always, well over the boiling point of water), even when the water/brine's temperature exiting the well exceeds the boiling point of water, it does not boil because of its very high pressure acquired via being circulated at a significant depth. As the pressure of a liquid working fluid increases, the boiling point of the working fluid also increases, as is well understood by those skilled in the art.
The geothermally supplied heat extracted from the primary working fluid by the refrigerant secondary working fluid both vaporizes and pressurizes the refrigerant/secondary working fluid, which may then be used to actuate a turbine and/or generator to produce electricity. The lower pressure refrigerant exiting the turbine/generator is then directed through a condenser (typically either air or water cooled), where the secondary working fluid changes phase back to a liquid and is then force-pumped (via a liquid refrigerant pump) back into the heat exchanger with the geothermally heated water to repeat the process.
However, in either a binary or an EGS geothermal power system, the power consumption required by the one or more primary working fluid water/brine circulating pumps is a significant parasitic electrical load that reduces the effective amount of electrical power generated by the system. The parasitic power draw of the liquid pump in the secondary above-ground power generating loop is also a negative factor.
In addition to the foregoing, there are other offshoot types of geothermal power production systems. For example, some systems use the heat available in hot (fully or partially molten) magma (in the 650° C. range), in volcanoes, or in hot dry rocks, or in geysers, etc.
Thus, the current binary geothermal power production plants require somewhat uniquely occurring circumstances to be economically viable, typically require at least two independent fluid loops (with typically at least two to three artificial and power parasitic liquid pumps required for system operation), and the geothermal “hot spots” generally necessary for economic viability are not universally available. Binary systems utilizing waste heat sources are more common, but either nuclear or fossil fuels are typically required as the primary heat source, both of which present well-understood environmental drawbacks.
The EGS designs may present additional environmental drawbacks by: producing seismic activity when fracturing hot dry rocks between deep wells; contributing to groundwater pollution when rock fracturing chemicals seep into aquifers; releasing significant quantities of carbon dioxide emissions (sometimes almost as much as that produced by a coal-fired power plant); and losing significant quantities of water on a continuous basis via condensing efforts at the surface and/or in the lowermost open loop portion of the deep wells deep (thereby depleting otherwise available natural water supplies).
While there are other known renewable and generally environmentally safe alternative energy sources, such as solar and wind, these alternative power sources are contingent upon uncontrollable conditions, such as the availability of sufficient sunshine or wind. Consequently, these more environmentally friendly alternatives are not reliable for day-to-day, consistent energy production. Even hydroelectric power production is dependent on adequate rainfall to maintain optimum design conditions.